<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<h3>
  结果：<span>

</span>
  <script>
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let s = document.querySelector("span");
    function f(x) {

      if(isNaN(i1.value)||isNaN(i2.value)){
        s.innerText="输入错误！";
        return;
      }
      switch(x){
          case 1:
              s.innerText = i1.value*1+i2.value*1;
              break;
          case 2:
              s.innerText=i1.value-i2.value;
              break;
          case 3:
              s.innerText=i1.value*i2.value;
              break;
          case 4:
              s.innerText = i1.value / i2.value;
              break;
      }
    }


  </script>
</h3>
</body>
</html>